This invention relates to semiconductor III-V alloy compounds, and more specifically to multispectral QWIPS.
The importance of semiconductor detectors is rapidly increasing along with progress in other opto-electronic fields, such as optical fiber communication, charge-coupled devices, and solid state lasers.
Thermal detectors and thermal imaging detector arrays are based on the local detection of infrared radiation emitted by a scene, and in the case of an imaging detector array allows the representation of the scene""s objects by the thermal gradients. This is because the infrared energy radiated by an object is proportional to its absolute temperature T0 and its emissivity xcex50. On the other hand, sources of stray infrared radiation (background, sky, Sun, other objects, etc.) having temperatures T1, T2, T3, etc. and emissivities xcex51,xcex52, xcex53, and so on, can add, after reflection, a disturbing radiative contribution to the intrinsic emission and reflectance of the object being imaged. The infrared detector or imager is sensitive to the sum of these reflected and emitted energies.
A multi-spectral infrared deflector is defined as a detector which is sensitive to more than one band of wavelengths of infrared radiation (usually infrared radiation is defined as wavelengths xcex in the range between xcex=1 xcexcm and xcex=50 xcexcm). Each band of wavelengths has a cut-on and cut-off wavelength, but is most commonly defined by a peak wavelength xcexp and a bandwidth xcex94xcex.
In the most favorable measurement case there are no stray reflections and the object is assumed to have a constant emissivity within the spectral intervals xcex94xcex1 and xcex94xcex2 (gray body). Then the energy emitted by the object itself depends only on the two quantities xcex50 and T0. However, a measurement in a single spectral band gives only one relationship and it is not possible to solve one equation for the two unknown variables. In this case the emissivity must be estimated or calibrated externally to the measurement. On the other hand, a thermal measurement system operating in two wavelength bands allows the setting up of two relationships (for the two unknowns) which ten determine the temperature T0 and emissivity xcex50 of the object. Thus only a bispectral measurement can give fast and accurate remote access to the thermal characteristics of an unknown object when the latter radiates as a gray body. Multi-spectral imaging (more then two wavelength bands detected) can be used to eliminate the effects of stray reflections.
Bispectral or multi-spectral imaging can provide information about the relative effects of emissivity and temperature when xcex50 is not constant, but is instead a function of wavelength. It is also possible to image or measure the temperature of two thermally different objects that would be indistinguishable ins single-band observation. In this case, if the two objects produce the same radiance in a spectral band xcex94xcex2. Thus, the thermal constant between the two objects in the band xcex94xcex2 can be used to increase the contrast of the overall image.
Practical applications of multi-spectral detectors include infrared xe2x80x9cheat seekingxe2x80x9d missiles which use infrared imaging arrays to locate and track the movement of the missile""s target. Often these targets are equipped with infrared countermeasures. These can include small decoys, such as flares or chafe, or powered decoy vehicles that can be deployed by an aircraft or warship to lure away the heat seeking missile. Countermeasures can also include infrared lamps and lasers intended to overload (blind) or burn the missile""s infrared detector. Decoys usually lure away the missile by providing a brighter (stronger) infrared signal at the wavelength to which the missile""s detector is most sensitive. In either case, a bispectral or multi-spectral detector is less likely to fail when opposed by such countermeasures. It can continue to track the target even if one waveband has been blinded/burned by a laser. And since most decoys cannot match the temperature and emissivity signature of the target across several wavebands, the spectral xe2x80x9cfingerprintxe2x80x9d of the target can be used to ignore decoy countermeasures.
When analyzing infrared images taken by a multi-spectral detector, one image is first generated for each waveband. In order to combine the images to improve the contrast, or to compute the temperature and emissivity, the signal for each pixel in the images must have come from the same source. If more than one detector is used, each detector will view a slightly different scene (parallax). This makes the pixel-to-pixel registration of the images computationally difficult and time consuming. For this reason monolithically integrated detectors, wherein the detectors are stacked on top of each other, are desirable.
An object, therefore, of the invention is a QWIP (Quantum Well Infrared Photodetector) consisting of a multiple quantum well structure grown on laP substrate for use as a sensor having multi-spectral detection.
A further object of the subject invention is a two-color QWIP structure for detection of two wavelengths in the range 3 less than xcex less than 5 microns.
A further object of the subject invention is a two-color QWIP structure for detection of two wavelengths, one in the range 3 less than xcex less than 5 and one in the range 8 less than xcex less than 12 microns.
A further object of the subject invention is a multi-spectral QWIP structure for simultaneous three-color detection of wavelengths in the ranges 3 less than xcex1 less than 5 microns, 8 less than xcex2 less than 12 microns.